1. Field of the Invention
The invention pertains to apparatus and method for tailored microwave heat treatment of low dielectric constant (k) organo-silicate glass (OSG) films deposited on substrates for semiconductor devices. More particularly, the invention pertains to burnout of post porosity plasma protection materials (e.g. PMMA and its variants) stuffed into the pores.
2. Description of Related Art
As integrated circuit feature sizes continue to shrink, new low dielectric constant (low-k) materials are needed to address problems with power consumption, signal propagation delays, and crosstalk between interconnects. One avenue to low-k dielectric films is the introduction of nanometer scale pores to lower the effective dielectric constant, which can therefore replace the dense silicon oxide insulator materials. Various materials and methods have been explored for deposition of porous organo-silicate glass (OSG) low-k films during the last 10-15 years. OSG materials have a silica-like backbone structure with a fraction of the Si—O bonds replaced with organic groups such as —CH3. This reduces overall dielectric constant of the material.
The relative dielectric constant of dense OSG is limited to k values greater than ˜2.7. It is expected that materials with even lower dielectric constants are needed for future generations of integrated circuits. To this end, a new class of OSG materials with porous structures introduced into the dense matrix has been successfully synthesized with k values as low as 2.0, which could be considered as leading candidates for use as interconnection dielectrics in emerging technologies.
The porous OSG films can be prepared by using a self-assembled technology to form nano-composite structures with controlled structure and physical properties. The spin-on low-k films are formed by condensing a hydrolyzed alkylated silica sol in the presence of a polymeric surfactant. This surfactant acts as a template to produce a regular porous structure as the film dries. Upon heat treatment the surfactant acts as a porogen and evaporates, thereby leaving behind a porous silica network with alkyl groups (e.g. methyl —CH3), which passivate the internal and external film surfaces.
The spin-on porous dielectric films have compositions similar to the popular plasma-enhanced chemical vapor deposition (PECVD) film materials. For the PECVD approach, a sacrificial porogen is added to the gaseous mix, which on heat treatment or processing is removed from the film and thus the pores are formed within the film. The porogen loading can be changed to alter the film porosity in the range of 7-45%. The heat treatment is usually performed at ˜400° C. or higher to completely remove the porogen, minimize the dielectric value k and simultaneously enhance the mechanical strength of the film.
However, successful implementation of these porous dielectrics structures poses numerous challenges. As compared to their dense counterparts, porous dielectrics are expected to have reduced cohesive strength and poorer adhesion with adjacent layers. They are more prone to the absorption of reactive chemicals during device fabrication, so much so that water can diffuse quite effectively into film stacks containing dielectric layer, even though the dielectric materials are usually hydrophobic. The moisture uptake could vary from ˜0.54 wt. % to 1.7 wt. % as the porosity is raised to 40 vol. %. The ingress of water into the dielectric negatively impacts both the mechanical integrity as well as electrical performance of the devices. Some isotope tracer diffusion and secondary ion mass spectroscopy experiments have revealed that water diffuses predominantly along the interface and not through the porous films. This result was attributed to the hydrophobic nature of the dielectric material and the hydrophilic character of the interfaces—hence the degradation of the interfacial adhesion.
Another challenge for these porous OSG low-k dielectrics is that they are susceptible to damage induced during etching and cleaning process, which degrades the dielectric and electrical properties and surface roughness. The plasma exposure breaks the weakly bonded organic terminal groups from the silica backbone while simultaneously densifying the porous silica-like damaged skin layer. Much effort is being dedicated, in the first place, to minimize the plasma damage. As a second choice, post-patterning remedies intended for dielectric restoration are being explored as well. Usually, trialkyl-substituted disilazanes or chlorosilanes such as hexamethyldisilane (HMDS) and trimethylchlorosilane (TMCS) or dimethylaminotrimethylsilane (DMATMS) and so on, are in use for dielectric restoration.
An approach to avoid this post porosity plasma or other processing (etching, chemical mechanical polishing) damage is to use a protective organic polymeric material (e.g. PMMA and its variants) which is temporarily stuffed into the pores. This approach strengthens the porous film to allow performing subsequent processes without damage. However, after the plasma processes have been performed this temporary material has to be removed or burnt out to recreate the pores and have a low k dielectric film.